1. Field of the Invention
The present invention generally relates to aerial devices such as ladders, towers, elevating platforms, and other such extension structures, commonly referred to herein as an aerial ladder tower or ladder tower, and more particularly, is concerned with an aerial ladder tower with pretensioned truss members which increases its live load capacity.
2. Description of the Prior Art
Aerial ladder towers are commercially available with multiple telescoping sections which extend in cantilever fashion from a support vehicle to considerable combined lengths. For instance, Simon Ladder Towers Incorporated (LTI) of Ephrata, Pennsylvania manufactures and sells aerial ladder tower models which extend to approximately 55, 75, 85 and 100 feet. Each ladder tower typically includes three or more telescoping sections with each section composed of two laterally spaced apart truss members interconnected by longitudinally spaced ladder rungs.
As it is telescoped out from a fully retracted position to a fully extended position, the aerial ladder tower is expected to deflect along its longitudinal extent due to its own weight ("dead load") and that of any persons standing on it ("live load"), with maximum deflection being at its free distal end where a work platform is commonly located. For example, in one LTI aerial ladder tower model adapted to extend to approximately 100 feet and designed to support an approximately 800 pound live load, a deflection of up to 18 inches at its distal end is to be expected due to its dead load alone. The ladder tower is also anticipated to deflect up to another 12 inches due to the application of the maximum live load of 800 pounds which results in a total maximum deflection of 30 inches for the combined dead load and maximum live load.
Although the aerial ladder tower is designed to be and in fact is structurally sound and safe even with such a large deflection, a problem is created by the deflection which is really one of perception of users. While not justified in their fears, users perceive that since the ladder tower has such a large deflection it must be unsafe. As a consequence, it would be desirable to provide reassurance to users by finding a way to appreciably reduce the deflection.
Generally speaking, there have been two techniques used in the prior art to strengthen single section ladders and multi-section extension ladders against bending and deflection -- by reinforcing the structures and by prestressing them. Representative of the prior art are the structures disclosed in U.S. Pat. Nos. to Smith (146,029), Mallory (785,901), Faulkner (1,167,988), Phelps (2,229,987), Troche (2,238,665 and 2,248,794) and Fischer et al (3,533,203). While many of the above-cited structures as well as others in the prior art probably operate reasonably well and generally achieve their objective under the limited range of operating conditions for which they were designed, none appears to suggest an appropriate solution to the aforementioned perception problem related to aerial ladder tower deflection.
Consequently, a need exists for an approach which will reduce aerial ladder tower deflection to a point where unjustified user concerns with safety because of such deflection will be substantially reduced, if not practically eliminated.